Pleura is a thin, transparent, two-layered membrane that covers the lungs and also lines the inside of the chest wall. The layer that covers the lungs is in close contact with the layer of the chest wall. Between these two thin layers is a small amount of fluid that lubricates each one as they slide over one another with each breath. Pleurisy is a painful condition which results when there is an abnormal amount of fluid between these two layers. The inflammation from this condition results in friction during inhalation and exhalation.